The Mega Cannon Sigma (Sigma Force) is Neji, Rivet, Veise, and Nutt. When they fuse they become the Super Mega Cannon Sigma. But in the show, Nutt was dead when they fused, so his power isn't accounted for.
At the bare minimum fusing adds all of their power together, which is impossible with a 1.25x boost.
Also, they're not all the same power, Nutt is weaker than Rivet and Veise who are weaker than Neji.
Neji: 2,000
Rivet: 1,500
Veise: 1,500
Nutt: 1,000
Super Mega Cannon Sigma: 5,000